1. Field of the Invention
The present invention relates generally to prescaling devices and methods and more particularly to a prescaling device and method useful for scaling an analog signal for use by an analog-to-digital converter. The invention is particularly suited for use in applications, such as spectrometry, in which the magnitude of an input analog signal varies over a relatively wide range and where automatic prescaling is desirable.
2. Description of the Prior Art
Analog-to-digital conversion techniques are widely used to provide an interface between an analog signal generator and a digital signal receiver or processor, such as a digital computer. Numerous analog-to-digital converters are known in the art, including dual slope converters and successive approximation converters.
Generally, an analog-to-digital converter (ADC) is adapted to receive an input analog signal within a fixed range. The ADC generates a plurality of output binary digits (bits), the value of which provides a binary representation of the input analog signal magnitude. The magnitude of the fixed range and the number of digits generated by the ADC determine the resolution for the ADC, that is, the increment or amount of analog signal represented by the least significant digit or bit of the ADC digital output. For example, the resolution of an ADC having a zero-to-ten volt input analog range and an eight digit or bit output is approximately 39 millivolts.
Because ADCs are generally adapted to operate within a fixed analog range, it is known to prescale or controllably attenuate or amplify the input analog signal and to apply the resulting prescaled signal to the ADC. Where the input analog signal is greater than the ADC range, prescaling prevents the ADC from operating in an overrange condition. Such a condition leads to the loss of data in the system and could damage the ADC if the input analog signal level is sufficiently high.
Also, where the input analog signal is less than the ADC fixed range, prescaling of the signal can increase the signal magnitude to correspond to a larger portion of the ADC range. Because the resolution of the ADC is generally fixed, such prescaling helps to improve the accuracy or precision of the resulting ADC digital output.
Various prescaling or ranging devices are known in the art. For example, prescaling may be done by using a plurality of attenuating resistors as shown in U.S. Pat. No. 3,399,349 to Davis or may be accomplished by varying the gain of an input amplifier stage as shown in U.S. Pat. Nos. 4,013,955 and 3,566,397 to Wagner and Walton, respectively. However, the precision resistive networks or networks which vary the gain of the amplifier require precision components which increase the cost and complexity of the device. Moreover, where switches are opened or closed to select the proper range, it may be necessary to allow a period of time to pass so that the device can stabilize after transients are introduced as a result of the switching process. Any such delay undesirably slows the prescaling process and thus the time for a complete measurement cycle.
The magnitude of prescaling required can be manually or automatically selected. Manual selection, however, is relatively slow and can result in inaccurate data. With respect to automatic prescaling, several such devices are illustrated in U.S. Pat. Nos. 3,703,001 to Hibbs, Jr.; 3,790,886 to Kurtin et al.; and 3,187,323 to Flood et al. However, such automatic prescaling devices generally require that an analog-to-digital measurement cycle be completed before an out-of-range condition is detected. Thus, one or more measurement cycles can be wasted before the proper magnitude of prescaling is selected. The wasted measurement cycles present a significant drawback particularly where the measurements are used in an automated system because data can be lost while selecting the required prescaling.
Where the input analog signal is relatively small, it is known to integrate the input signal over a plurality of predetermined time intervals. One such system is disclosed in U.S. Pat. No. 3,525,093 to Marshall. At the end of each interval, an integrator which integrates the input signal is reset toward zero. A value stored by a counter is varied according to the amount that the integrator was reset. Such a system can be considered as a form of an analog-to-digital converter.
A system as disclosed in Marshall, however, suffers from several difficulties. For example, it is relatively slow since it requires a number of time periods in order to accumulate a meaningful result. Moreover, the system may not provide accurate results because the integrator may have a significant charge left at the end of the last predetermined interval. Also, the range of the input signal is limited by the time interval selected and the maximum amount that the integrator can be reset at the end of a time interval.
Thus, there is a need for a prescaling device and method which is adapted to operate automatically but which does not waste measurement time in determining the proper amount of prescaling required. There is also a need for a prescaling device and method which operates relatively quickly and precisely yet which can accommodate a wide range of input signal levels.